A method of determining or predicting a value Px of a property (e.g., octane number) of a material X or a property of a product of a process from said material or yield of said process for example a blending, separation, or chemical (e.g., polymerisation) process, which method comprises measuring the absorption Dix of said material at more than one wavelength in the region 600-2600 nm, comparing the said signals indicative of said absorptions or a derivative thereof with signals indicative of absorptions Dim or derivatives thereof at the same wavelength for a number of standards S in a bank for which the said property or yield P is known, and choosing from the bank at least one standard Sm with property Pm said standard having the smallest average value of the absolute difference at each wavelength i between the signal for the material and the signal for the standard Sm to obtain Px, with averaging of said properties or yields Pm when more than one standard Sm is chosen.